The Nobel Prize for Physics for the year 1937 will today be
delivered to Dr. C.J. Davisson and Professor G.P. Thomson for
their discovery of the interference phenomena arising when
crystals are exposed to electronic beams.

The study of the dispersion and diffraction phenomena produced by
beams of electrons impinging on crystal surfaces was begun
already in 1922 by Davisson and his collaborator Kunsman. These
investigations soon obtained special actuality in connection with
the theory of mechanical waves pronounced in 1923 by the Nobel
Prize winner Prince
de Broglie. According to this theory material particles are
always linked with a system of travelling waves, a
«wave-packet», forming the constituent parts of matter
and determining its movements. We might get a popular picture of
the relation between a material particle and the associated
mechanical waves, if we assume space filled with wave systems
travelling with somewhat different velocities. In general these
waves neutralize one another, but at certain points it happens
that a great number of waves are in such a position as to
reinforce one another and form a marked wave crest. This wave
crest then corresponds to a material particle. Since, however,
the waves travel with different velocity they will part from one
another, and the wave crest disappears to be found again at a
nearby point. The material particle has moved. The wave crest
will thus travel, but the velocity with which this is done is
quite different from the one with which the underlying wave
systems move. The material particle in general moves at right
angles to the surfaces of the mechanical waves, just as a ray of
light is, as a rule, directed at right angles to the surface
planes of the light waves.

The theory of de Broglie derived from analogies between the laws
ruling the movement of a material particle and those applying in
the case of the passage of a ray of light.

A great number of phenomena observed in optics can neither be
explained nor described by the aid of rays of light, and this
holds true especially of the diffraction and dispersion phenomena
produced when light passes through a narrow slit or by a sharp
edge. To explain those phenomena it is necessary to have recourse
to the hypothesis of the propagation of light by means of
waves.

In recent times, the existence of diffraction and interference
phenomena has settled a dispute regarding the nature of a certain
radiation. This time the X-rays were concerned. The question was
whether these rays consist of particles ejected with great
velocity or of electromagnetic waves.

The mechanical grids utilized for studying interference phenomena
in optics let through the X-rays without diffraction. This might
be due to the wavelength of these rays being so short that the
grids became too wide. The Nobel Prize winner von Laue then got the
ingenious idea to use as grids, crystals, the regularly arranged
atoms of which could serve as diffraction centres. It was also
stated that the X-rays in those grids gave rise to diffraction
and interference phenomena; the X-rays consequently consisted of
waves.

The mechanical waves of de Broglie now correspond to the waves of
light and the path of the material particle to the passage of the
ray of light.

In his theory de Broglie found a simple relation between the
velocity of the material particle and the wavelength of the
«wave-packet» associated with this particle. The
greater the velocity of the particle the shorter is the
wavelength. If the velocity of the particle is known, it is then
possible to calculate, by means of the formula indicated by de
Broglie, the wavelength and vice versa.

The theory of de Broglie of mechanical waves and the development
of wave mechanics have been of radical importance to modern atom
theory.

It is therefore quite natural that this revolutionary theory
should become the object of assiduous research as to its
consequences and of efforts to prove experimentally the existence
of mechanical waves.

As has already been mentioned, Davisson had, together with his
collaborator Kunsman, in the year before the theory of de Broglie
was presented, started a series of experiments on the diffraction
phenomena produced when a beam of electrons impinges with a
certain velocity on the surface of a crystal. These experiments
which were continued during the following years, gave, however,
at the beginning results rather strange and hard to explain,
probably due to the great experimental difficulties connected
with the apparatus arrangement. In 1928, however, the
investigations met with such a success that Davisson and his
collaborator Germer were able to present the incontestable
evidence, reached by experiments, of the existence of mechanical
waves and of the correctness of the theory of de Broglie. Four
months later Professor Thomson, who had been studying the same
problem independently of Davisson and by the aid of a different
apparatus equipment for his experiments, also confirmed de
Broglie's theory.

For their experiments Davisson and Germer availed themselves of a
cubic nickel crystal. Here the atoms are symmetrically arranged
in planes parallel to the end surfaces of the crystal, the atoms
forming a quadratic network in the planes. However, as radiation
surface was not used the end plane of the cube but the triangular
plane obtained, if an angle of the cube is symmetrically cut off.
The atoms in this plane form a triangular network.

A minute bundle of electrons of determined velocity were emitted
perpendicularly upon this plane. If we assume the incoming
electrons replaced by mechanical waves, the planes of which are
thus parallel to the surface of the crystal, these mechanical
waves will strike the atoms lying in the surface simultaneously,
and these atoms as centres will, in their turn, emit new
mechanical waves in all directions. The waves going out in a
certain direction can be studied and measured by the aid of a
so-called Faraday chamber placed in this direction. In this
chamber the mechanical waves cause the same effect as the
corresponding electrons. In order to describe better how the
outgoing radiation arises, let us suppose the receiving device
placed so as to capture the waves going out parallel to the
crystal plane and at right angles to one of the sides of the
triangle. Parallel to this side the atoms lie in parallel rows
with a certain distance between the rows, this distance having
been determined beforehand by the aid of X-ray investigations.
Every row now emits its wave. But the waves from the inner rows
arrive later, due to the longer way they have to pass to reach
the edge of the triangle. As a rule an irregular system of waves
is thus obtained in which the waves neutralize each other, and
consequently no outgoing wave is produced. If on the other hand
the mechanical waves should be of such a wavelength that the
distance between the rows of atoms becomes equal to the
wavelength or to a multiple thereof, all the outgoing waves will
be in phase and reinforce one another. In this case a wave system
going out in the direction indicated is obtained or, if
preferable, a bundle of outgoing electronic beams.

The experiments now showed at what velocities of the incoming
electrons outgoing beams are produced, and these have, according
to what has been stated above, a wavelength equal to the distance
between the rows of atoms. Since thus the wavelength of the
mechanical waves had been found and since the velocity of the
corresponding electron was known, it was possible to check the
formula of de Broglie. Davisson found that the theory agreed with
the experiments except for 1 to 2%. Davisson and Germer examined
the reflection of the electronic beams in various directions and
obtained results which agreed with the wave theory.

During his experiments Davisson used electron beams with rather a
low velocity corresponding to the one obtained when an electron
is made to pass a voltage between 50 and 600 volts.

Thomson, on the other hand, for his experiments availed himself
of swift electrons with a velocity corresponding to voltages
between 10,000 and 80,000 volts. These swift electrons have
afterwards proved to be of great use in connection with studies
on the structure of matter.

For his experiments Thomson made use of exceedingly thin films of
celluloid, gold, platinum, or aluminium. He made the electron
beam fall perpendicularly upon the film and examined the
diffraction figures produced on a fluorescent screen placed
behind the film, or else had them reproduced on a photographic
plate. The thickness of the films used for the experiments
amounted to between 1/10,000 and 1/100,000 of a millimetre. Such
a film now consists of innumerable small crystals of various
directions. In accordance with what the theory indicates, there
is generally obtained on the screen a series of concentric rings
corresponding to the various directions of the planes in a
crystal where a regularly arranged network of atoms can be found.
From the diametre of a ring, the wavelength of the mechanical
wave can be determined, and to make possible the production of a
ring this wavelength must be in accordance with the spacing of
the planes in the system of planes to which the ring corresponds.
A similar method has been applied previously by Debye-Scherrer
for X-rays analysis of the structure of crystals. Thomson found
very good agreement with the theory of de Broglie. He further
found that a magnetic field influencing the beams having passed
the film produced a lateral movement of the image on the screen,
which shows that these beams consist of bundles of
electrons.

For the above-mentioned experiments electrons have been employed
as matter; later investigations have confirmed the correctness of
de Broglie's theory also for such cases where beams of molecules,
atoms, and atom nuclei have been used.

The purpose of the said experiments was to verify the theory of
de Broglie, and to this end was utilized the knowledge of the
arrangement of the atoms in a crystal, this knowledge having been
previously acquired as a result of investigations by means of
X-rays. Now that the law of de Broglie has become known and
acknowledged, the opposite way has been taken. From the law of de
Broglie we know the wavelength of the mechanical waves
accompanying an electronic beam with a certain velocity of the
electrons. By changing this velocity we can then obtain
electronic waves with known wavelengths. By application of one or
the other of the investigation methods mentioned above we can
find the distances between the various atom planes within the
crystal and thus also the structure of the crystal. The procedure
is here the same as the one previously applied to determine the
structure of crystals by means of X-rays. We have thus obtained a
new method for such investigation, but the two methods have found
very different fields of application due to the different nature
of the beams employed. The X-rays are pure electromagnetic rays
like the rays of light, and they therefore influence but slightly
the atoms of the crystal, and owing to this circumstance easily
traverse the crystal structure. From the same reason the
diffracted rays are comparatively feeble, and many hours'
exposure is therefore required to record X-ray diagrams. The
mechanical waves, on the other hand, are associated with
electrical charges which are very strongly influenced by the
charges of the crystal atoms. The mechanical waves will therefore
be rapidly absorbed in the crystal, and the interference figures
obtained only come from an exceedingly thin surface layer. In
return the intensity of the diffracted or reflected bundles of
electrons becomes very great, and the time of exposure required
is consequently extremely short, in many cases only a fraction of
a second. These properties of the electronic beams make them an
exceedingly important complement to the X-rays as far as
researches on the structure of matter are concerned. At the
important investigations of the structure of surfaces good
results can be attained only by the new method, since the images
of the X-rays are influenced by the matter lying behind the
surface layer. By the aid of electronic beams it has thus been
possible to explain how the structure of the surfaces of metals
is changed by various mechanical, thermal, or chemical treatment.
It has also been possible to ascertain the properties of thin
layers of gases and powder. On account of the rapid exposure
which the electronic beams permit, we can follow the course of
the changes occurring in connection with the oxidization of
metals and also observe the corrosion phenomenon in iron and
steel for various thermal treatment as well as the chemical
process ensuing when metals are attacked by corrosive substances.
The intensity of radiation is so great that one can easily carry
out investigations of the structure of crystals with a mass of
less than a millionth of a gram. This has made it possible to
discover in certain substances exceedingly minute crystalline
structures, which it would not have been possible to find by
means of X-ray investigations.

It would bring us too far here to enter upon the multitude of
experimental results furnished by the method with electronic
beams, especially as new fields of application of the electron
beam are incessantly being opened up within the spheres of
physical and chemical research.

Dr. Davisson. When you found that electron
beams touching crystals give rise to phenomena of diffraction and
interference, this signified in itself a discovery that widened
essentially our knowledge of the nature of electrons. But this
discovery has proved to be of still greater importance. Your
researches concerning these phenomena resulted in your presenting
the first positive, experimental evidence of the wave nature of
matter. The investigation methods that you and Professor Thomson
have elaborated and the further research work carried out by both
of you have provided science with a new, exceedingly important
instrument for examining the structure of matter, an instrument
constituting a very valuable complement to the earlier method
which makes use of the X-ray radiation. The new investigations
have already furnished manifold new, significant results within
the fields of physics and chemistry and of the practical
application of these sciences.

On behalf of the Royal Swedish Academy of Sciences I congratulate
you on your important discoveries, and I now ask you to receive
your Nobel Prize from the hands of His Majesty.

The Royal Swedish Academy of Sciences much
regrets that Professor Thomson has not had the opportunity of
being present on this occasion to receive in person his Nobel
Prize. The prize will now instead be delivered to His Excellency
the Minister of Great Britain.

Your Excellency. Permit me to request you
to receive on behalf of Professor Thomson the Nobel Prize for
Physics from the hands of His Majesty.